A. Field of Invention
The present invention is related to wireless communication system configuration, and more particularly, to a method and system of spread spectrum modulation.
B. Description of Related Art
In a typical wireless communication system, an area is divided geographically into a number of cell sites, each defined by one or more radio frequency (RF) radiation patterns from a respective base transceiver station (BTS) antenna. Most cell sites are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS. The radiation pattern of each sector in a cell site produces an air interface that defines a respective coverage area.
Cell sites of the typical wireless communication system are operated by one or more wireless service providers (WSP). A WSP is a company that provides telecommunications through RF signals rather than (or in addition to) through end-to-end wire communication. A WSP may transmit signals using one or more carrier frequencies. A carrier frequency (or simply carrier) is a transmitted electromagnetic pulse or wave at a steady frequency of alternation on which information can be imposed by increasing signal strength, varying the frequency, varying the wave phase, or other means. When a cell site is operated by WSPs with multiple carrier frequencies, the cell site may be considered to have a multiple of its number of sectors. For instance, if a cell site is divided into three sectors by directional antenna elements, and the cell site is operated by a WSP that transmits signals over four carrier frequencies, then the cell site may effectively have twelve sectors, three operating at each of the four carrier frequencies.
A code division multiple access (CDMA) wireless communication system may have cell sites operated with multiple carrier frequencies. On a given frequency in a CDMA system, multiple communication channels exist, each distinguished by a “Walsh code.” Walsh code is a term used for a digital modulation code that distinguishes individual signals on the RF carrier frequency being transmitted. There are 64 possible Walsh codes and each code is 64 bits long.
The communication channels of the CDMA system that the Walsh code identifies include a pilot channel (Walsh code 0), a sync channel (Walsh code 32), a number of paging channels (Walsh codes 1 possibly through 7), and a number of traffic channels (any remaining Walsh codes). Pilot, sync, and paging channels are “overhead,” or system channels that are used to establish system timing and BTS identity with an MS and to manage the signal transmissions between an MS and a BTS. Traffic channels carry individual user conversations or data.
The pilot signal continuously repeats a simple signal at high power levels so that MSs can easily locate the BTS. The pilot signal is usually the strongest of the 64 Walsh channels, and is on Walsh code 0. Paging channels can be found on one or more of Walsh codes 1 to 7. These channels are used to notify MSs of incoming calls from the system and to handle the MSs' responses in order to assign the incoming calls to traffic channels. The synchronization, or sync channel is found on Walsh code 32. This channel carries a single repeating message with timing and system configuration information from the wireless communication system.
In active communication, a Walsh codes are used to distinguish BTSs from one another. When an MS detects suitable signal strength of a Walsh code (from the pilot channel) of a BTS, the MS may begin communication with the BTS that has that Walsh code. The BTS may also determine which MS to communicate with by monitoring pilot signals of MSs in the wireless system. The BTS may send signals to the MS on a forward communication link (BTS-MS communication) and the MS may send signals to the BTS on a reverse communication link (MS-BTS communication).
In the CDMA system, when an MS is present in a given sector of a cell site, communications between the MS and the BTS of the cell site may be encoded by an encoding scheme of the sector, regardless of the carrier frequency being used. An encoding scheme may be a signaling protocol used to distinguish communications between MSs and BTSs. Each sector of a cell site (e.g., sectors A, B, C) may use a different encoding scheme in order to distinguish between the other sectors of the cell site (i.e., sector A may distinguish from sectors B and C, etc.).
In normal operation of the typical wireless communication system, when an MS is within a sector of a cell site defined by a BTS, the MS may communicate with the BTS through the air interface produced by the sector. If the MS moves into a new sector of a new BTS, through communication with a base station controller (BSC), the MS may simply communicate with the new BTS through the new sector in order to complete a handoff from one BTS to the next. Therefore, when the MS moves from a coverage area of one BTS to another, a “soft handoff” may occur because the MS transitions from an initial sector of a cell site to a different sector of a different cell site.
Similarly, when an MS is within an initial sector of a BTS and the MS moves from the initial sector of the BTS into a new sector of the same BTS, the MS may then communicate with the BTS through the new sector. Through communication with the BTS, the MS may simply switch to use the new sector in order to complete the handoff from the initial sector to the next. Therefore, when the MS moves from one sector to another within the same cell site, a “softer handoff” occurs.
Soft handoff and softer handoff between an MS and a BTS may interrupt signal transmission and reception between the MS and the BTS. In addition, the MS may be required to constantly monitor pilot signals from the BTS that the MS is in communication with as well as pilot signals from other nearby BTSs in order to determine if a soft handoff or a softer handoff is necessary. This can become inefficient and consume a large amount of power within the MS. Consequently, it is desirable to reduce handoff between an MS and a BTS.